Element magnesium (Mg), being abundant on the earth's surface, light in weight, tough and well capable of being cast, has wide applications as parts of wheels, airplanes, and cellular phones. Because of being vulnerable to corrosion, however, Mg gets easily corroded with generation of hydrogen as it is immersed in an acidic solution, an alkaline solution or saline; there have been a variety of rust preventives developed for the purpose of protecting the surfaces of metal substrates such as Mg and Mg alloy substrates.
For instance, Patent Publication 1 discloses how to manufacture rustproof, anticorrosive iron materials without recourse to harmful chromium compounds, and Patent Publication 2 discloses a hexavalent chromium-free, surface-treated metal sheet and how to produce it.
Tannin, a sort of polyphenols, has been used as rust preventives since before to form a stable film typically on the surface of zinc as well known in the art (see Non-Patent Publication 1). However, tannin has only limited use because of being hardly soluble in an organic solvent. To provide a solution to this, it has been known in the art that at least some of hydroxyl groups contained in the tannin molecule are substituted by an alkyl ether or an alkyl ester to obtain a water-insoluble tannic acid derivative (Patent Publication 3).